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An Attempt to Evaluate Plant Available P in Western Nigerian Soils Under Shifting Cultivation

In temperate regions, means of predicting P availability to crops have centered around the use of soil tests that extract mainly inorganic P. In tropical areas where organic matter declines rapidly in soils recently brought under cultivation, crops probably benefit more from the release of P from organic forms than from inorganic forms. Therefore, this study was designed to investigate through greenhouse and laboratory studies 1) the relationship between soil organic matter and different inorganic P extractants, 2) the relationship between soil organic matter and some other soil variables, (cation exchange capacity-CEC, percent clay, cations, soil pH), and 3) whether organic matter can be used as a general guide line for recommending P fertilization on recently cleared soils. Two crops of rice (Oryra sativa L.) were grown on surface soil samples collected from 90 different locations with different cropping histories and covering different ecological zones in western Nigeria. Inorganic P in these soil samples was extracted with seven extractants. The soil samples were also analyzed for clay, organic matter, organic P, CEC, exchangeable K, Ca, and Mg, and soil pH. The following correlation studies were made: 1) simple correlation between organic matter, Bray's P₁, available K, exchangeable Ca and Mg, CEC, and clay, 2) simple and multiple correlation of P uptake with extractable P, soil organic matter, and different P extractants, and 3) simple and multiple correlations of yield with P uptake, Bray's P₁, organic matter, and organic P. No significant correlations were found between soil clay and Bray's P₁ or CEC, but there were significant correlations between organic matter, and Bray's P₁ and CEC indicating that some of the extractable P may be associated with the soil organic matter or with organic P mineralization. Soils with less than 3% soil organic matter showed no significant correlations between extractable P and soil organic matter while soils having between 3 and 5% organic matter showed a significant correlation (r = 0.62, significant at the 0.01 level). Six of the seven extractants showed significant positive correlation between organic matter and extractable P namely, Saunders (r = 0.51, significant at the 0.01 level), Olsen's (r = 0.26, significant at the 0.05 level), Bray's P₁ (r = 0.36, significant at the 0.05 level), Bray's P₄ (0.5 H NH₄F + 0.1 HCl) (r = 0.50, significant at the 0.01 level), Bray's P₂ (r = 0.43, significant at the 0.05 level) and North Carolina (r = 0.25, significant at the 0.05 level). The correlation with the water extraction was not significant. There was better correlation between extractable P determined before cropping and plant uptake by the first crop, than between extractable P determined after harvesting the first crop and uptake by the second crop. In some cases, the extractable P before the second crop was higher than the initial extractable P which indicates that substantial mineralization of organic P may have occurred during the first cropping period. The correlation coefficients for yield vs. P test were low indicating that none of the extractants effectively predicted plant uptake of P. It is therefore concluded that none of the extractants tested extracts organic P, that some factors other than those measured are affecting availability of P, and that organic P is important to plants in the tropics and may be a good predictor of P availability in soils that are high in organic matter but not for soils low in organic matter.

Key Words: ‘OS-6,’ • Organic P • Tropical soils • Oryza sativa L.

Received for publication October 18, 1975.